This invention relates to a detector for diffracted electrons for analyzing the signal intensity of diffraction spots with an electron diffraction detector.
When a crystal surface is irradiated by an electron beam, the beam is generally diffracted by the surface. The diffracted electrons form a pattern, but different diffraction patterns are formed if the direction of the crystal is changed with respect to the direction of incidence of the electron beam.
The fact that the positions of diffraction spots comprising a diffraction image (or pattern) change with the scanning by the electron beam indicates that the directions of crystal grains are different at each portion of the sample surface. Thus, by examining whether a diffraction spot appears or not at a certain position as different areas of a sample surface are scanned, it is possible to detect the shape and distribution of crystal grains in the direction which satisfies corresponding conditions for the diffraction.
There have therefore been known detectors for diffracted electrons using this principle to detect the shapes and distributions of crystal grains in a given sample. FIG. 3 shows a first example of such a prior art detector. As shown in FIG. 3, an electron beam emitted from an electron gun 31 is incident on a sample 36, and the electron beam diffracted by the surface of the sample 36 forms a diffraction pattern on a fluorescent screen 32. In order to study the differences in the diffraction pattern at different positions on the sample surface, the electron beam is moved around by means of a scan control circuit 33. A diffraction spot is selected by moving a detector 34 while observing the diffraction pattern formed on the fluorescent screen 32 either directly or by means of a TV camera. An image for showing the distribution of crystal grains can be obtained by displaying on a CRT 35 an on-line image of signal from the detector in synchronism with the electron beam scan.
FIG. 4 shows another prior art detector for diffracted electrons disclosed, for example, in Japanese Patent Publication Tokkai 4-204361. As illustrated in FIG. 4, an electron beam narrowly restricted by an electron gun 41 is made incident on a sample 46 nearly parallel to its surface, and the electrons which are diffracted by this surface are caused to collide with a fluorescent screen 42. The electron beam is converted into optical signals on the screen 42 and forms a diffraction pattern. A camera tube 44 scans this diffraction pattern according to scan signals received from a scan signal circuit 47, and image signals thus obtained are displayed on a cathode ray tube (CRT) 45. In order to set the position of a diffraction spot, an operator specifies the position of any picture element on the diffraction pattern by using a mouse 49 and a CPU 48. The scan timing for image signals corresponding to the picture element at the specified position is obtained by the CPU 48. Numeral 50 indicates a gate which is opened at this scan timing such that bright signals of the specified diffraction spot are extracted from image signals of the camera tube 44 and sent to another CRT 51. The CRT 51 uses these brightness signals in synchronism with scan signals from a scan control circuit 43 to thereby display an image showing the distribution of crystal grains.
There are problems, however, associated with such prior art detectors for diffracted electrons. With a detector of the type described above with reference to FIG. 3, for example, it is difficult to accurately match the detector with the position of a diffraction spot. In order to "catch" a desired diffraction spot, the detector 34 of FIG. 3 is usually moved in front of the fluorescent screen 32. During such a maneuver, either the detector 34 itself or a mechanism for moving it (not shown) is likely to cover the diffraction pattern formed on the fluorescent screen 32, interfering with its observation, say, by a TV camera (not shown) and making it difficult to align the detector 34 with a diffraction spot or to check its alignment.
With a detector of the type described above with reference to FIG. 4, on the other hand, the scanning time per picture element by the electron beam is limited by the time of scanning one image screen by the camera tube. In other words, since signals corresponding to the brightness of a specified diffraction spot are displayed by extracting signals at a rate of timing corresponding to the position of the specified picture element and synchronizing these extracted signals with the scanning of the sample surface by the electron beam, the speed of scanning the sample by the electron beam is set by the timing of extracting the image signals. The timing of extracting image signals is the same as the rate of scanning one image screen by the camera, and it is once every 1/30 second in the case of an ordinary TV camera. In other words, the time for the electron beam to scan one picture element is limited by this rate, and the scanning speed of the electron beam could not be increased.